Centrifugal pumps are well known and widely used in a variety of industries to pump fluids or liquid/solid components of fluid mixtures. Centrifugal pumps, particularly those of the pitot tube type, generally comprise a pump housing or pump casing and a rotor assembly positioned within the pump housing which rotates by means of connection to a drive unit. Centrifugal pumps of the pitot tube type have a fluid inlet and a fluid discharge positioned relative to the rotor assembly for introducing fluid into the rotor assembly and for removing fluid from the rotor assembly, respectively.
In conventional pitot tube pumps, the fluid inlet and fluid discharge are positioned in parallel orientation on the same side of the pump housing, in a side-by-side or concentric arrangement. Fluid is directed through the pump inlet into the rotor chamber, and as the rotor assembly rotates, the fluid is directed toward the interior peripheral surface of the rotor chamber as a result of centrifugal forces. Fluid moving within the rotor assembly is intercepted by the inlet of the stationary pitot tube, and fluid moves through the inlet of the pitot tube, through the pitot tube arm and toward the discharge outlet of the pump.
Typical centrifugal pumps of the pitot tube type are disclosed in U.S. Pat. No. 3,822,102 to Erickson, et al., U.S. Pat. No. 3,960,319 to Brown, et al., U.S. Pat. No. 4,161,448 to Erickson, et al., U.S. Pat. No. 4,280,790 to Crichlow, U.S. Pat. No. 4,332,521 to Erickson and U.S. Pat. No. 4,674,950 to Erickson. In the pumps disclosed in the referenced patents, the fluid inlet and discharge outlet are positioned on the same side of the pump casing in coaxial and concentric alignment. In other pitot tube constructions, the inlet into the rotor assembly may be positioned on one side of the rotor assembly, opposite the position of the pitot tube assembly, thus positioning the inlet and the discharge in co-axial or parallel axial arrangement, but not concentric arrangement.
In all pitot tube pump configurations, elevated pressures are realized at or near the axial center of the rotor assembly. These elevated pressures are observed more readily in pump configurations that employ a dual inlet or double bladed pitot tube assembly as opposed to a single bladed pitot tube assembly, although elevated pressures are observed in both pump configurations. This elevated pressure is thought to be caused, in large part, by fluid displacement caused by the position of the pitot tube assembly in the fluid chamber of the rotor assembly. Other influences may also increase pressure within the rotor assembly.
As a result of the elevated pressures near the axial center of the rotor assembly, various problems are experienced in the operation of the pump that reduce pump efficiencies. The most notable problem caused by elevated pressures near and at the axial center of the rotor assembly is high axial thrust, which has deleterious effects of the bearing system of the pump. Additionally, elevated pressures at or near the axial center of the rotor assembly influence high recirculation in concentric inlet-discharge arrangements in conventional pitot tube pumps, and exert elevated pressure at or on the discharge seal in co-axial, non-concentric pitot tube arrangements.
It has been demonstrated that the magnitude of pressure exerted at or near the axial center of the rotor assembly is affected by rotor assembly speed and pitot tube design (e.g., double blade versus single blade). Thus, the displacement of fluid within the fluid chamber of the rotor assembly by the pitot tube appears to have a significant influence on the elevated pressures that are observed.